A number of devices have been described that make it possible to transport powder material over long distances. Most of them call upon a fluidized bed transport technology. U.S. Pat. No. 3,268,264 described a process used to transport powder material in which a device is installed between the supply zone and the zone to be supplied, said device being a closed device including at least one substantially horizontal conveyor, which comprises a lower duct designed for the circulation of a gas and an upper duct designed for the circulation of powder material, said lower duct and said upper duct being separated by a porous wall that said gas can pass through and said lower duct being provided with at least one gas feed pipe. The lower duct is supplied with gas at a pressure that allows said powder material in said upper duct to be fluidized. Such a device, designed to transport coal in powder form to supply thermal power plants, describes a system of partitioning in the lower duct that makes it possible to vary the fluidization pressure along the air chute. In the operating conditions described in this document the fluidized material flows in turbulent state, with a fairly high fluidization gas flow rate.
French patent FR 2534891 (ALUMINIUM PECHINEY) also discloses a device making it possible to potentially fluidify powder material with a low fluidization gas flow rate so that the powder material is maintained in a state known as a “hyperdense phase” With such a device, substantially horizontal piping can be used to transport said material in hyperdense phase from a supply zone to a zone to be supplied. The device in FR 2354891 is distinguished from that in U.S. Pat. No. 3,268,264 primarily by the fact that the upper duct is provided with at least one balancing column the open top end of which communicates with the outside of said closed device and which is filled with powder material. The height to which the column is filled balances the pressure prevailing in the upper duct. The powder material is thereby placed in a state of potential fluidization: the gas+powder material mixture behaves like a liquid and remains motionless as long as no powder material is consumed. As soon as a vacuum is created in the zone to be supplied successive micro-landslides occur from said vacuum as far as the storage zone, so that the powder material circulates with a controlled flow, just sufficient for the needs of the zone to be supplied. Air is supplied at low pressure in comparison to the other dense phase transport processes and the movement of the gas, like that of the particles; occurs at low speed, which makes it possible to limit wear of the equipment and attrition of the product. The balancing column is preferably assembled vertically. Its section preferably lies between S/20 and S/200, S being the entire surface of said porous wall.
In European patent EP 1086035 (ALUMINIUM PECHINEY), the above process is improved by arranging the upper duct so that a gas bubble can be formed under pressure in its top part. In this way, operation is more stable: the risks of the conveyor, called “air chute”, becoming completely jammed when there is no degassing, or incomplete degassing, in one of the balancing columns are reduced. Typically, walls forming a barrier and “solidifying” the bubble in the upper zone of said upper duct are fitted into the upper zone of said upper duct. The height of these barriers is lower than half the height of the upper duct. Typically, it is about one tenth of said height. Preferably, the upper zone of the upper duct is arranged so that a given bubble, bounded by said barrier walls, is associated with a balancing column of the device.
The device for conveyance on a potential fluidization hyperdense bed, as described in the previous patents, is used on a large scale, in particular to supply the tanks of recent plants performing igneous electrolysis of aluminum. The advantages of this device are well-known:                continuous supply to the tanks, making it possible to keep the hoppers permanently full,        little system maintenance required,        relatively low air pressures required for fluidization (0.1 bar as compared with 6 bar for pneumatic transport in dense phase),        low-speed movement of alumina, reducing wear of the equipment and attrition or agglomeration of the product.        
But, while it has all the above-mentioned advantages, this device may have some disadvantages if certain special precautions are not taken:                a less-than-optimal consumption of fluidization gas, and therefore of energy,        fly-offs, i.e. significant recycling of alumina, via the balancing columns,        a risk of particle size segregation by preferential fly-off of the finest particles.        
In addition, in an electrolysis workshop, a great number (several dozen) of zones need to be supplied from only one storage zone. Moreover, the distance between the storage zone and the zone to be supplied may be great (several hundred meters). To meet with these requirements, the applicant proposed the device illustrated in EP-B-0179055, made up of a series of cascading conveyors: a primary conveyor connecting the storage zone to a series of secondary conveyor s, each one assigned to a tank and provided with side nozzles supplying hoppers integrated into the superstructure of the tank. This makes up a network of air chutes making it possible to transfer the alumina as a hyperdense bed over several hundred meters, typically between 400 and 800 meters. However, the applicant noted that at such distances, it was sometimes difficult to avoid the appearance of segregation phenomena in certain places.
Over and above this problem, encountered specifically in the context of the transport of alumina, the applicant attempted to better define the conditions in which a powder material resulting from a homogeneous mixture of particles of different particle sizes, or even a material also resulting from a homogeneous mixture of particles of different densities, can circulate in such air chutes without segregation occurring. In other words, the applicant sought to define the optimum conditions for conveying such powder material by potential fluidization, by which conditions said material has the same particle size distribution or the same density distribution when it arrives in the zone to be supplied.